Is it necessary to clean the tongue and how to do that?

For me, yes it is since baby is too young to rise his own mouth after feeding/meal. Just as Hui Qun has mentioned, it's for hygiene purposes and to remove any sort of milk build up that might have accumulate on his tongue. I use Nuk Oral Wipes too, but I bought them in bulk during baby fairs. It's approx. $99 per carton and it lasted for a pretty long time since I only use these when I am outside with my son. Otherwise at home, just a gauze will do as Hui Qun had explained.